This invention relates to a device which detects the brightness of an original document to be scanned and controls its exposure to light according to the detected level of brightness of the document and more particularly to such a device for which the timing of controlling the exposure is accurately set with respect to the position at which brightness of the document is detected.
There have been original document scanning devices such as copiers and telecopiers having the automatic exposure controlling capability, or the ability to automatically adjust the exposure to an appropriate level according to the brightness of the original document to be scanned. Such devices typically control the level of exposure by using a beam of light from a source to irradiate the document to be scanned, receiving the beam reflected from the document surface by a light receiving element and controlling the brightness of the source such that its signal will have a predetermined value. Although the exposure of a photosensitive body or an image sensor can thus be adjusted automatically to an optimum level, there arises a problem of delay in the control circuit which controls the exposure on the basis of a signal from the light receiving element.
One of the methods of overcoming this problem in the case of a high-speed copier, for example, has been to define a brightness detecting point and to control the exposure appropriately on the basis of the document brightness measured at the brightness detecting point. In the case of a copier with variable magnification, however, the scanning speed changes when the magnification is changed. As a result, the timing of response by the exposue controlling device may become incorrect, depending on the magnification, although an interval of a fixed distance is set as explained above.